1. Field of the Invention
The present invention relates to a method of manufacturing a suspension structure, and more particularly, to a method of manufacturing a suspension structure using a sacrificial layer, dry etching process, and lift off process.
2. Description of the Prior Art
Micro-electromechanical systems (MEMS) are modern technology, which coordinate electrical circuits and mechanics. MEMS have been applied to fabricate devices with both electrical circuits and mechanical elements, such as pressure sensors, accelerators, and micro-microphones. The suspended structure is applied in many MEMS devices such as a MEMS switch, inkjet nozzle, and micro-microphones, etc., but the method of manufacturing a suspension structure according to the prior art still has many limitations.
Please refer to FIG. 1 to FIG. 4. FIG. 1 to FIG. 4 shows the method of manufacturing a suspension structure by using wet etching process according to the prior art. As shown in FIG. 1, a metal deposition process is performed on a semiconductor substrate 12 to form an adhesive layer. Before a copper (Cu) metal layer is deposited as a structure layer, the metal deposition process deposits a titanium (Ti) metal layer 14 as an adhesive layer between the semiconductor substrate 12 and the Cu metal layer 16, and the Ti metal layer 14 is deposited on the semiconductor substrate 12 by using an electron gun evaporation system in the metal deposition process. Next, the Cu metal layer 16 is deposited on the Ti metal layer 14, and there are two methods for the metal deposition process of the Cu metal layer 16. The first method is directly using the electron gun evaporation system to deposit the Cu metal layer 16 with thickness of about 1 micrometer. The second method is using the electron gun evaporation system to deposit a seed layer first, and then deposit the Cu metal layer 16 on the seed layer by electroplating process. Next, a passivation layer 18 is deposited to protect the Cu metal layer 16. For example, a metal layer of nickel (Ni), chromium (Cr), Ti, or aurum (Au) is deposited as the passivation layer 18 by using the electron gun evaporation system. Then, a photoresist layer 20 is coated on the passivation layer 18 by using a spin coating machine.
As shown in FIG. 2, a first photoresist pattern 22 is formed by a photolithography process on the photoresist layer 20, and then a portion of the passivation layer 18, the Cu metal layer 16, and the Ti metal layer 14 that are not protected by the first photoresist pattern 22 are removed in sequence by a etching process. Next, the first photoresist pattern 22 is removed, and a second photoresist pattern 24 is formed on the passivation layer 18 and the semiconductor substrate 12, as shown in FIG. 3.
At last, a wet etching process using the KOH etching liquid, for example, is performed to form a hole 26 on the semiconductor substrate 12 to form the suspension structure 10, and the second photoresist pattern 24 is removed, as shown in FIG. 4.
The method of manufacturing a suspension structure according to the prior art has some disadvantages as follows. First, the KOH etching liquid can etch aluminum (Al), and few materials can serve as an etching mask for the KOH etching liquid, only LPCVD Si3N4 or low stress Si3N4 for example. Therefore, the prior art has to use specific metal that is not able to be etched by the KOH etching liquid, or form extra passivation layers and adhesive layers above and under the structure layer respectively. Second, since the prior art uses the wet etching process to remove the semiconductor substrate to form a hole in the semiconductor substrate, therefore the suspension structure will be affected by the surface tension of the etching liquid which results in a broken structure layer, or the suspension part sticking to the substrate surface. Third, it is very difficult for the prior art methods to design and manufacture the suspension structure with various shapes by adjusting the process parameters.